Scalable distributed storage architecture

ABSTRACT

Techniques are disclosed for providing a file system interface for an object store intended to support simultaneous access to objects stored in the object store by multiple clients. In accordance with one method, an abstraction of a root directory to a hierarchical namespace for the object store is exposed to clients. The object store is backed by a plurality of physical storage devices housed in or directly attached to the plurality of host computers and internally tracks its stored objects using a flat namespace that maps unique identifiers to the stored objects. The creation of top-level objects appearing as subdirectories of the root directory is enabled, wherein each top-level object represents a separate abstraction of a storage device having a separate namespace that can be organized in accordance with any designated file system.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is related to the following commonly assigned,co-pending applications: “Distributed Policy-Based Provisioning andEnforcement for Quality of Service” (Ser. No. ______, Attorney DocketNo. VMW/0271 (B281)), “Load Balancing of Resources” (Ser. No. ______,Attorney Docket No. VMW/0275 (B285)), and “Virtual Disk Blueprint for aVirtualized Storage Area Network” (Ser. No. ______, Attorney Docket No.VMW/0301 (B492)), each of which was filed on Aug. 26, 2013. Each relatedapplication is incorporated by reference herein in its entirety.

BACKGROUND

Distributed systems allow multiple clients in a network to access a poolof shared resources. For example, a distributed storage system allows acluster of host computers to aggregate local disks (e.g., SSD, PCI-basedflash storage, SATA, or SAS magnetic disks) located in or attached toeach host computer to create a single and shared pool of storage. Thispool of storage (sometimes referred to herein as a “datastore” or“store”) is accessible by all host computers in the cluster and may bepresented as a single namespace of storage entities (such as ahierarchical file system namespace in the case of files, a flatnamespace of unique identifiers in the case of objects, etc.). Storageclients in turn, such as virtual machines spawned on the host computersmay use the datastore, for example, to store virtual disks that areaccessed by the virtual machines during their operation. Because theshared local disks that make up the datastore may have differentperformance characteristics (e.g., capacity, input/output per second orIOPS capabilities, etc.), usage of such shared local disks to storevirtual disks or portions thereof may be distributed among the virtualmachines based on the needs of each given virtual machine.

This approach provides enterprises with cost-effective performance. Forinstance, distributed storage using pooled local disks is inexpensive,highly scalable, and relatively simple to manage. Because suchdistributed storage can use commodity disks in the cluster, enterprisesdo not need to invest in additional storage infrastructure. However, onechallenge that arises relates to developing a mechanism to efficientlytrack where objects are stored across the commodity disks in the clusteras well as how to efficiently access them when needed. For example,while utilizing a flat namespace may provide a simplistic and efficientmeans to store and retrieve objects, it does not provide enoughflexibility to create hierarchical relationships between objects thatmay be useful in organizing objects in a manner that is compatible withthe existing interfaces of clients or that otherwise satisfies thedifferent storage requirements of different clients. For example, manypre-existing environments that could leverage such a scalable objectstore (e.g., applications, storage management tools, virtualizationhypervisors, etc.) may require that the object store provide a certainhierarchical file system based storage interface. One example is vSphereHypervisor from VMware, Inc. which stores virtual machine metadata inthe form of files in a hierarchical file system. Furthermore, becausethe datastore is shared among the cluster of host computers which maysimultaneously access the same data stored therein, any file system thatis used to manage the single namespace provided by the datastore needsto have mechanisms for concurrency control. Current distributed orclustered file systems typically provide some form of concurrencycontrol. However, due to limitations in their inherent design, suchcurrent file systems typically have limitations on the number ofsimultaneous “clients” (e.g., host computers or virtual machines thataccess the file system) they can support. If a current file system usedto manage the datastore has limits on the number of clients that cansimultaneously access it, then the utility of a highly scalabledatastore can plateau, even if additional commodity storage can beeasily added to the datastore to increase its capacity, since noadditional clients can be added to access such additional storage.

SUMMARY

One embodiment of the present disclosure relates to a method forproviding a file system interface for an object store intended tosupport simultaneous access to objects stored in the object store bymultiple clients. In accordance with the method, an abstraction of aroot directory to a hierarchical namespace for the object store isexposed to clients. The object store is backed by a plurality ofphysical storage devices housed in or directly attached to the pluralityof host computers and internally tracks its stored objects using a flatnamespace that maps unique identifiers to the stored objects. Thecreation of top-level objects appearing as subdirectories of the rootdirectory is enabled, wherein each top-level object represents aseparate abstraction of a storage device having a separate namespacethat can be organized in accordance with any designated file system.

By layering a hierarchical namespace that can be used by clients whileusing a flat namespace to internally store and access objects,techniques herein are able to offer a framework that supports a largescalable clustered file system using a distribute of commodity storageresources. For example, providing the capability to create top-levelobjects representing file systems that contain file objects that mayultimately be accessed by clients, the foregoing object store offershigher scalability than existing distributed or clustered file systemsbecause it is not confined, for example, by any limitations on thenumber of simultaneous clients inherent in the design of any particulardistributed clustered file system (e.g., VMware VMFS, NFS, etc.)configured for any particular file system object in the object store.That is, implementing a clustered file system on top of an object storehas scalability advantages, because different parts of the clusteredfile system can be placed on different objects such that scalabilityrequirements from particular clients need to be achieved only by thesubset of such clients that access data in a certain sub-space of thenamespace which, in turn, resides on a certain object.

Other embodiments include, without limitation, a computer-readablemedium that includes instructions that enable a processing unit toimplement one or more aspects of the disclosed methods as well as acomputer system having a processor, memory, and modules configured toimplement one or more aspects of the disclosed methods.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 illustrates an example computing environment, according to oneembodiment.

FIG. 2 illustrates an example hierarchical structure of objectsorganized within an object store that represent a virtual disk,according to one embodiment.

FIG. 3 illustrates components of a VSAN module, according to oneembodiment.

FIG. 4 illustrates a method flow diagram for creating a virtual diskobject based on a defined storage policy, according to one embodiment.

FIG. 5 illustrates the handling of an I/O operation originating from aVM, according to one embodiment.

DETAILED DESCRIPTION

FIG. 1 illustrates a computing environment 100, according to oneembodiment. As shown, computing environment 100 is a software-based“virtual storage area network” (VSAN) environment that leverages thecommodity local storage housed in or directly attached (hereinafter, useof the term “housed” or “housed in” may be used to encompass both housedin or otherwise directly attached) to host servers or nodes 111 of acluster 110 to provide an aggregate object store 116 to virtual machines(VMs) 112 running on the nodes. The local commodity storage housed in orotherwise directly attached to the nodes 111 may include combinations ofsolid state drives (SSDs) 117 and/or magnetic or spinning disks 118. Incertain embodiments, SSDs 117 serve as a read cache and/or write bufferin front of magnetic disks 118 to increase I/O performance. As furtherdiscussed below, each node 111 may include a storage management module(referred to herein as a “VSAN module”) in order to automate storagemanagement workflows (e.g., create objects in the object store, etc.)and provide access to objects in the object store (e.g., handle I/Ooperations to objects in the object store, etc.) based on predefinedstorage policies specified for objects in the object store. For example,because a VM may be initially configured by an administrator to havespecific storage requirements for its “virtual disk” depending itsintended use (e.g., capacity, availability, IOPS, etc.), theadministrator may define a storage profile or policy for each VMspecifying such availability, capacity, TOPS and the like. As furtherdescribed below, the VSAN module may then create an “object” for thespecified virtual disk by backing it with physical storage resources ofthe object store based on the defined policy

A virtualization management platform 105 is associated with cluster 110of nodes 111. Virtualization management platform 105 enables anadministrator to manage the configuration and spawning of VMs on thevarious nodes 111. As depicted in the embodiment of FIG. 1, each node111 includes a virtualization layer or hypervisor 113, a VSAN module114, and hardware 119 (which includes the SSDs 117 and magnetic disks118 of a node 111). Through hypervisor 113, a node 111 is able to launchand run multiple VMs 112. Hypervisor 113, in part, manages hardware 119to properly allocate computing resources (e.g., processing power, randomaccess memory, etc.) for each VM 112. Furthermore, as described furtherbelow, each hypervisor 113, through its corresponding VSAN module 114,provides access to storage resources located in hardware 119 (e.g., SSDs117 and magnetic disks 118) for use as storage for virtual disks (orportions thereof) and other related files that may be accessed by any VM112 residing in any of nodes 111 in cluster 110. In a particularembodiment, vSphere Hypervisor from VMware, Inc. (VMware) may beinstalled on nodes 111 as hypervisor 113 and vCenter Server from VMwaremay be used as virtualization management platform 105.

In one embodiment, VSAN module 114 is implemented as a “VSAN” devicedriver within hypervisor 113. In such an embodiment, VSAN module 114provides access to a conceptual “VSAN” 115 through which anadministrator can create a number of top-level “device” or namespaceobjects that are backed by object store 116. In one common scenario,during creation of a device object, the administrator may specify aparticular file system for the device object (such device objectshereinafter also thus referred to “file system objects”). For example,in one embodiment, each hypervisor 113 in each node 111 may, during aboot process, discover a /vsan/ root node for a conceptual globalnamespace that is exposed by VSAN module 114. By, for example, accessingAPIs exposed by VSAN module 114, hypervisor 113 can then determine allthe top-level file system objects (or other types of top-level deviceobjects) currently residing in VSAN 115. When a VM (or other client)attempts to access one of the file system objects, hypervisor 113 maydynamically “auto-mount” the file system object at that time. In certainembodiments, file system objects may further be periodically“auto-unmounted” when access to objects in the file system objects ceaseor are idle for a period of time. A file system object (e.g.,/vsan/fs_name1, etc.) that is accessible through VSAN 115 may, forexample, be implemented to emulate the semantics of a particular filesystem such as VMware's distributed or clustered file system, VMFS,which is designed to provide concurrency control among simultaneouslyaccessing VMs. Because VSAN 115 supports multiple file system objects,it is able provide storage resources through object store 116 withoutbeing confined by limitations of any particular clustered file system.For example, many clustered file systems (e.g., VMFS, etc.) can onlyscale to support a certain amount of nodes 111. By providing multipletop-level file system object support, VSAN 115 overcomes the scalabilitylimitations of such clustered file systems.

As described in further detail in the context of FIG. 2 below, a filesystem object, may, itself, provide access to a number of virtual diskdescriptor files (e.g., .vmdk files in a vSphere environment, etc.)accessible by VMs 112 running in cluster 110. These virtual diskdescriptor files contain references to virtual disk “objects” thatcontain the actual data for the virtual disk and are separately backedby object store 116. A virtual disk object may itself be a hierarchicalor “composite” object that, as described further below, is furthercomposed of “component” objects (again separately backed by object store116) that reflect the storage requirements (e.g., capacity,availability, IOPs, etc.) of a corresponding storage profile or policygenerated by the administrator when initially creating the virtual disk.As further discussed below, each VSAN module 114 (through a clusterlevel object management or “CLOM” sub-module, in embodiments as furtherdescribed below) communicates with other VSAN modules 114 of other nodes111 to create and maintain an in-memory metadata database (e.g.,maintained separately but in synchronized fashion in the memory of eachnode 111) that contains metadata describing the locations,configurations, policies and relationships among the various objectsstored in object store 116. This in-memory metadata database is utilizedby a VSAN module 114 on a node 111, for example, when an administratorfirst creates a virtual disk for a VM as well as when the VM is runningand performing I/O operations (e.g., read or write) on the virtual disk.As further discussed below in the context of FIG. 3, VSAN module 114(through a document object manager or “DOM” sub-module, in oneembodiment as further described below) traverses a hierarchy of objectsusing the metadata in the in-memory database in order to properly routean I/O operation request to the node (or nodes) that houses (house) theactual physical local storage that backs the portion of the virtual diskthat is subject to the I/O operation.

FIG. 2 illustrates an example hierarchical structure of objectsorganized within object store 116 that represent a virtual disk,according to one embodiment. As previously discussed above, a VM 112running on one of nodes 111 may perform I/O operations on a virtual diskthat is stored as a hierarchical or composite object 200 in object store116. Hypervisor 113 provides VM 112 access to the virtual disk byinterfacing with the abstraction of VSAN 115 through VSAN module 114(e.g., by auto-mounting the top-level file system object correspondingto the virtual disk object, as previously discussed, in one embodiment).For example, VSAN module 114, by querying its local copy of thein-memory metadata database, is able to identify a particular filesystem object 205 (e.g., a VMFS file system object in one embodiment,etc.) stored in VSAN 115 that stores a descriptor file 210 for thevirtual disk (e.g., a .vmdk file, etc.). It should be recognized thatthe file system object 205 may store a variety of other files consistentwith its purpose, such as virtual machine configuration files (e.g.,.vmx files in a vSphere environment, etc.) and the like when supportinga virtualization environment. In certain embodiments, each file systemobject may be configured to support only those virtual diskscorresponding to a particular VM (e.g., a “per-VM” file system object).

Descriptor file 210 includes a reference to composite object 200 that isseparately stored in object store 116 and conceptually represents thevirtual disk (and thus may also be sometimes referenced herein as avirtual disk object). Composite object 200 stores metadata describing astorage organization or configuration for the virtual disk (sometimesreferred to herein as a virtual disk “blueprint”) that suits the storagerequirements or service level agreements (SLAs) in a correspondingstorage profile or policy (e.g., capacity, availability, IOPs, etc.)generated by an administrator when creating the virtual disk. Forexample, in the embodiment of FIG. 2, composite object 200 includes avirtual disk blueprint 215 that describes a RAID 1 configuration wheretwo mirrored copies of the virtual disk (e.g., mirrors) are each furtherstriped in a RAID 0 configuration. Composite object 225 may thus containreferences to a number of “leaf” or “component” objects 220,corresponding to each stripe (e.g., data partition of the virtual disk)in each of the virtual disk mirrors. The metadata accessible by VSANmodule 114 in the in-memory metadata database for each component object220 (e.g., for each stripe) provides a mapping to or otherwiseidentifies a particular node 111, in cluster 110 that houses thephysical storage resources (e.g., magnetic disks 118, etc.) thatactually store the stripe (as well as the location of the stripe withinsuch physical resource).

FIG. 3 illustrates components of a VSAN module 114, according to oneembodiment. As previously described, in certain embodiments, VSAN module114 may execute as a device driver exposing an abstraction of a VSAN 115to hypervisor 113. Various sub-modules of VSAN module 114 handledifferent responsibilities and may operate within either user space 315or kernel space 320 depending on such responsibilities. As depicted inthe embodiment of FIG. 3, VSAN module 114 includes a cluster levelobject management (CLOM) sub-module 325 that operates in user space 315.CLOM sub-module 325 generates virtual disk blueprints during creation ofa virtual disk by an administrator and ensures that objects created forsuch virtual disk blueprints are configured to meet storage profile orpolicy requirements set by the administrator. In addition to beingaccessed during object creation (e.g., for virtual disks), CLOMsub-module 325 may also be accessed (e.g., to dynamically revise orotherwise update a virtual disk blueprint or the mappings of the virtualdisk blueprint to actual physical storage in object store 116) on achange made by an administrator to the storage profile or policyrelating to an object or when changes to the cluster or workload resultin an object being out of compliance with a current storage profile orpolicy.

In one embodiment, if an administrator creates a storage profile orpolicy for a composite object such as virtual disk object 200, CLOMsub-module 325 applies a variety of heuristics and/or distributedalgorithms to generate virtual disk blueprint 215 that describes aconfiguration in cluster 110 that meets or otherwise suits the storagepolicy (e.g., RAID configuration to achieve desired redundancy throughmirroring and access performance through striping, which nodes' localstorage should store certain portions/partitions/stripes of the virtualdisk to achieve load balancing, etc.). For example, CLOM sub-module 325,in one embodiment, is responsible for generating blueprint 215describing the RAID 1/RAID 0 configuration for virtual disk object 200in FIG. 2 when the virtual disk was first created by the administrator.As previously discussed, a storage policy may specify requirements forcapacity, IOPS, availability, and reliability. Storage policies may alsospecify a workload characterization (e.g., random or sequential access,I/O request size, cache size, expected cache hit ration, etc.).Additionally, the administrator may also specify an affinity to VSANmodule 114 to preferentially use certain nodes 111 (or the local diskshoused therein). For example, when provisioning a new virtual disk for aVM, an administrator may generate a storage policy or profile for thevirtual disk specifying that the virtual disk have a reserve capacity of400 GB, a reservation of 150 read TOPS, a reservation of 300 write TOPS,and a desired availability of 99.99%. Upon receipt of the generatedstorage policy, CLOM sub-module 325 consults the in-memory metadatadatabase maintained by its VSAN module 114 to determine the currentstate of cluster 110 in order generate a virtual disk blueprint for acomposite object (e.g., the virtual disk object) that suits thegenerated storage policy. As further discussed below, CLOM sub-module325 may then communicate the blueprint to its corresponding distributedobject manager (DOM) sub-module 340 which interacts with object space116 to implement the blueprint by, for example, allocating or otherwisemapping component objects (e.g., stripes) of the composite object tophysical storage locations within various nodes 111 of cluster 110.

In addition to CLOM sub-module 325 and DOM sub-module 340, as furtherdepicted in FIG. 3, VSAN module 114 may also include a clustermonitoring, membership, and directory services (CMMDS) sub-module 335that maintains the previously discussed in-memory metadata database toprovide information on the state of cluster 110 to other sub-modules ofVSAN module 114 and also tracks the general “health” of cluster 110 bymonitoring the status, accessibility, and visibility of each node 111 incluster 110. The in-memory metadata database serves as a directoryservice that maintains a physical inventory of the VSAN environment,such as the various nodes 111, the storage resources in the nodes 111(SSD, magnetic disks, etc.) housed therein and thecharacteristics/capabilities thereof, the current state of the nodes 111and there corresponding storage resources, network paths among the nodes111, and the like. As previously discussed, in addition to maintaining aphysical inventory, the in-memory metadata database further provides acatalog of metadata for objects stored in object store 116 (e.g., whatcomposite and component objects exist, what component objects belong towhat composite objects, which nodes serve as “coordinators” or “owners”that control access to which objects, quality of service requirementsfor each object, object configurations, the mapping of objects tophysical storage locations, etc.). As previously discussed, othersub-modules within VSAN module 114 may access CMMDS sub-module 335(represented by the connecting lines in FIG. 3) for updates to learn ofchanges in cluster topology and object configurations. For example, aspreviously discussed, during virtual disk creation, CLOM sub-module 325accesses the in-memory metadata database to generate a virtual diskblueprint, and in order to handle an I/O operation from a running VM112, DOM sub-module 340 accesses the in-memory metadata database todetermine the nodes 111 that store the component objects (e.g., stripes)of a corresponding composite object (e.g., virtual disk object) and thepaths by which those nodes are reachable in order to satisfy the I/Ooperation.

As previously discussed, DOM sub-module 340, during the handling of I/Ooperations as well as during object creation, controls access to andhandles operations on those component objects in object store 116 thatare stored in the local storage of the particular node 111 in which DOMsub-module 340 runs as well as certain other composite objects for whichits node 111 has been currently designated as the “coordinator” or“owner.” For example, when handling an I/O operation from a VM, due tothe hierarchical nature of composite objects in certain embodiments, aDOM sub-module 340 that serves as the coordinator for the targetcomposite object (e.g., the virtual disk object that is subject to theI/O operation) may need to further communicate across the network with adifferent DOM sub-module 340 in a second node 111 (or nodes) that servesas the coordinator for the particular component object (e.g., stripe,etc.) of the virtual disk object that is stored in the local storage ofthe second node 111 and which is the portion of the virtual disk that issubject to the I/O operation. If the VM issuing the I/O operationresides on a node 111 that is also different from the coordinator of thevirtual disk object, the DOM sub-module 340 of the node running the VMwould also have to communicate across the network with the DOMsub-module 340 of the coordinator. In certain embodiments, if the VMissuing the I/O operation resides on node that is different from thecoordinator of the virtual disk object subject to the I/O operation, thetwo DOM sub-modules 340 of the two nodes may to communicate to changethe role of the coordinator of the virtual disk object to the noderunning the VM (e.g., thereby reducing the amount of networkcommunication needed to coordinate I/O operations between the noderunning the VM and the node serving as the coordinator for the virtualdisk object).

DOM sub-modules 340 also similarly communicate amongst one anotherduring object creation. For example, a virtual disk blueprint generatedby CLOM module 325 during creation of a virtual disk may includeinformation that designates which nodes 111 should serve as thecoordinators for the virtual disk object as well as its correspondingcomponent objects (stripes, etc.). Each of the DOM sub-modules 340 forsuch designated nodes is issued requests (e.g., by the DOM sub-module340 designated as the coordinator for the virtual disk object or by theDOM sub-module 340 of the node generating the virtual disk blueprint,etc. depending on embodiments) to create their respective objects,allocate local storage to such objects (if needed), and advertise theirobjects to their corresponding CMMDS sub-module 335 in order to updatethe in-memory metadata database with metadata regarding the object. Inorder to perform such requests, DOM sub-module 340 interacts with a logstructured object manager (LSOM) sub-module 350 that serves as thecomponent in VSAN module 114 that actually drives communication with thelocal SSDs and magnetic disks of its node 111. In addition to allocatinglocal storage for component objects (as well as to store other metadatasuch a policies and configurations for composite objects for which itsnode serves as coordinator, etc.), LSOM sub-module 350 additionallymonitors the flow of I/O operations to the local storage of its node111, for example, to report whether a storage resource is congested.

FIG. 3 also depicts a reliable datagram transport (RDT) sub-module 345that delivers datagrams of arbitrary size between logical endpoints(e.g., nodes, objects, etc.), where the endpoints may potentially beover multiple paths. In one embodiment, the underlying transport is TCP.Alternatively, other transports such as RDMA may be used. RDT sub-module345 is used, for example, when DOM sub-modules 340 communicate with oneanother, as previously discussed above to create objects or to handleI/O operations. In certain embodiments, RDT module 345 interacts withCMMDS module 335 to resolve the address of logical endpoints dynamicallyin order to maintain up-to-date location information in the in-memorymetadata database as well as to create, remove, or reestablishconnections based on link health status. For example, if CMMDS module335 reports a link as unhealthy, RDT sub-module 345 may drop theconnection in favor of a link in better condition.

FIG. 4 illustrates a method flow diagram for creating a virtual diskobject based on a defined storage policy, according to one embodiment.For example, in step 400, an administrator may interact with a userinterface of virtual management platform 105 to create a virtual diskhaving capacity, availability and IOPS requirements (e.g., the definedstorage policy). In one embodiment, virtual management platform 105 maythen request a “master” node 111 to create an object for the virtualdisk in step 405. In step 410, such a master node 111 may generate avirtual disk blueprint through its CLOM sub-module 325 in VSAN module.As previously discussed, CLOM sub-module 35 generates a virtual diskblueprint for the creation of a virtual disk object (e.g., a compositeobject) based on the status of cluster 110 as determined by consultingthe in-memory metadata database of CMMS sub-module 335. The virtual diskblueprint may identify a particular node that should serve as thecoordinator or owner of the virtual disk object. In step 415, the DOMsub-module 340 of the master node 111 may the request the DOM sub-module340 of the identified node to create the virtual disk object. In step420, the DOM sub-module 340 of the identified node receives the requestand creates the virtual disk object, by, for example, communicating withits corresponding the LSOM sub-module 350 to persistently store metadatadescribing the virtual disk object in its local storage. In step 425,the DOM sub-module 340, based on the virtual disk object blueprint,identifies those others nodes in cluster 110 that have been designatedto serve as the coordinator or owner for any component objects in thevirtual disk blueprint. The DOM sub-module 340 communicates (e.g., usingits RTP sub-module 345) with the DOM sub-modules 340 of the other nodesthat will serve as coordinators for the component objects and store thedata backing such component objects in their local storage. When suchDOM sub-modules 340 receive a request from the DOM sub-module 340 of thecoordinator of the virtual disk object to create their respectivecomponent objects, they, in turn in step 430, communicate with theirrespective LSOM modules 350 to allocate local storage for the componentobject (and its related metadata). Once such component objects have beencreated, their DOM sub-modules 340 advertise the creation of thecomponents to the in-memory metadata database of its CMMS sub-module 335in step 435. In step 440, in turn, the DOM sub-module 340 for thecoordinator of the virtual disk object also advertises its creation toits CMMDS sub-module 335 to update the in-memory metadata database andultimately transmits an acknowledgement to the administrator (e.g., viathe master node communications back to virtual management platform 105).

FIG. 5 illustrates the handling of an I/O operation originating from aVM, according to one embodiment. When a VM running on a particular nodeperforms I/O operations to its virtual disk, the VM's guest operatingsystem, in step 500, transmits an I/O operation request intended for itsvirtual disk (through a device driver of the guest operating system)which, in step 505, is received by hypervisor 113 and ultimatelytransmitted and transformed through various layers of an I/O stack inhypervisor 113 to DOM sub-module 340 of VSAN module 114. In step 510,the I/O request received by DOM sub-module 340 includes a uniqueidentifier for an object representing the virtual disk that DOMsub-module 340 uses to identify the coordinator node of the virtual diskobject by accessing the in-memory metadata database of CMMS sub-module335 (in certain embodiments, accessing the in-memory metadata databaseto look up a mapping of the identity of the coordinator node to theunique identifier occurs only when the virtual disk object is initiallyaccessed, with such mapping persisting for future I/O operations suchthat subsequent lookups are not needed). Upon identifying thecoordinator node for the virtual disk object, the DOM sub-module 340 ofthe node running the VM communicates (e.g., using its RTP sub-module345) with the DOM sub-module 340 of the coordinator node to request thatit perform the I/O operation in step 515. As previously discussed, incertain embodiment, if the node running the VM and the node serving ascoordinator of the virtual disk object are different, the two DOMsub-modules will communicate to update the role of the coordinator ofthe virtual disk object to be the node of the running VM. Upon thecoordinator's receipt of the I/O request, in step 520, its DOMsub-module identifies (e.g., by again referencing the in-memory metadatadatabase, in certain embodiments) those coordinator nodes for theparticular component objects (e.g., stripes) of the virtual disk objectthat are subject to the I/O operation. For example, if the I/O operationspans multiple stripes (e.g., multiple component objects) of a RAID 0configuration, DOM sub-module 340 may split the I/O operation andappropriately transmit correspond I/O requests to the respectivecoordinate nodes for the relevant component objects that correspond tothe two stripes. In step 525, the DOM sub-module of the coordinator nodefor the virtual disk object requests that the DOM sub-modules for thecoordinator nodes of the identified component objects perform the I/Ooperation request and, in step 530, the DOM sub-modules of suchcoordinator nodes for the identified component objects interact withtheir corresponding LSOM sub-modules to perform the I/O operation in thelocal storage resource where the component object is stored.

Although one or more embodiments have been described in some detail forclarity of understanding, it will be apparent that certain changes andmodifications may be made within the scope of the claims. Accordingly,the described embodiments are to be considered as illustrative and notrestrictive, and the scope of the claims is not to be limited to detailsgiven herein, but may be modified within the scope and equivalents ofthe claims. For example, although a number of foregoing describedembodiments describe virtual machines as the clients that access thevirtual disks provided by the VSAN module, it should be recognized thatany clients, such as a cluster of non-virtualized host servers and/ornon-virtualized applications running therein may similarly utilize theVSAN module in alternative embodiment. Similarly, alternativeembodiments of the VSAN module may enable creation of high level storageobjects other than virtual disks, such as, without limitation, RESTobjects, files, file systems, blob (binary large objects) and otherobjects. Similarly, while VSAN module 114 has been generally depicted asembedded in hypervisor 113, alternative embodiments may implement VSANmodule separate from hypervisor 113, for example as a special virtualmachine or virtual appliance, a separate application or any other“pluggable” module or driver that can be inserted into computingplatform in order to provide and manage a distributed object store.Similarly, while the foregoing embodiments have referred to RAIDconfigurations as one technique to organize a blueprint, it should berecognized that other configurations may be utilized in otherembodiments, including, without limitation, using erasure codes andother similar techniques. While descriptions herein have discussed using“unique identifiers” to reference objects in the objects, it should berecognized that techniques to generate unique identifiers (hashes, etc.)may not necessarily be guaranteed to generate truly unique identifiersand therefore certain embodiments may further implement techniques forhandling name collisions in case there are instances where identifiersfor objects are not truly unique. In one such embodiment, in addition tothe purported unique identifier, an additional administrator or userspecified identifier is also assigned to an object during its creation(or otherwise mapped to the object's unique identifier). When there is aname collision due to a pre-existing object having the same uniqueidentifier as a newly created object, the administrator oruser-specified name can be transformed into the newly created object'suser identifier.

The various embodiments described herein may employ variouscomputer-implemented operations involving data stored in computersystems. For example, these operations may require physical manipulationof physical quantities usually, though not necessarily, these quantitiesmay take the form of electrical or magnetic signals where they, orrepresentations of them, are capable of being stored, transferred,combined, compared, or otherwise manipulated. Further, suchmanipulations are often referred to in terms, such as producing,identifying, determining, or comparing. Any operations described hereinthat form part of one or more embodiments may be useful machineoperations. In addition, one or more embodiments also relate to a deviceor an apparatus for performing these operations. The apparatus may bespecially constructed for specific required purposes, or it may be ageneral purpose computer selectively activated or configured by acomputer program stored in the computer. In particular, various generalpurpose machines may be used with computer programs written inaccordance with the teachings herein, or it may be more convenient toconstruct a more specialized apparatus to perform the requiredoperations.

The various embodiments described herein may be practiced with othercomputer system configurations including hand-held devices,microprocessor systems, microprocessor-based or programmable consumerelectronics, minicomputers, mainframe computers, and the like.

One or more embodiments may be implemented as one or more computerprograms or as one or more computer program modules embodied in one ormore computer readable media. The term computer readable medium refersto any data storage device that can store data which can thereafter beinput to a computer system computer readable media may be based on anyexisting or subsequently developed technology for embodying computerprograms in a manner that enables them to be read by a computer.Examples of a computer readable medium include a hard drive, networkattached storage (NAS), read-only memory, random-access memory (e.g., aflash memory device), a CD (Compact Discs), CD-ROM, a CD-R, or a CD-RW,a DVD (Digital Versatile Disc), a magnetic tape, and other optical andnon-optical data storage devices. The computer readable medium can alsobe distributed over a network coupled computer system so that thecomputer readable code is stored and executed in a distributed fashion.

In addition, while described virtualization methods have generallyassumed that virtual machines present interfaces consistent with aparticular hardware system, the methods described may be used inconjunction with virtualizations that do not correspond directly to anyparticular hardware system. Virtualization systems in accordance withthe various embodiments, implemented as hosted embodiments, non-hostedembodiments, or as embodiments that tend to blur distinctions betweenthe two, are all envisioned. Furthermore, various virtualizationoperations may be wholly or partially implemented in hardware. Forexample, a hardware implementation may employ a look-up table formodification of storage access requests to secure non-disk data.

Many variations, modifications, additions, and improvements arepossible, regardless the degree of virtualization. The virtualizationsoftware can therefore include components of a host, console, or guestoperating system that performs virtualization functions. Pluralinstances may be provided for components, operations or structuresdescribed herein as a single instance. Finally, boundaries betweenvarious components, operations and data stores are somewhat arbitrary,and particular operations are illustrated in the context of specificillustrative configurations. Other allocations of functionality areenvisioned and may fall within the scope of one or more embodiments. Ingeneral, structures and functionality presented as separate componentsin exemplary configurations may be implemented as a combined structureor component. Similarly, structures and functionality presented as asingle component may be implemented as separate components. These andother variations, modifications, additions, and improvements may fallwithin the scope of the appended claims(s). In the claims, elementsand/or steps do not imply any particular order of operation, unlessexplicitly stated in the claims.

1. (canceled)
 2. A method for providing access to a plurality ofphysical storage devices, comprising: executing a virtual computinginstance on a respective node of a plurality of nodes, wherein therespective node includes a hypervisor, a plurality of physical storagedevices, random access memory, a computer processing unit, and a storagemanagement module; exposing, to the virtual computing instance, anabstraction of a datastore, wherein the datastore is backed by theplurality of physical storage devices housed in or directly attached toa plurality of nodes including a first physical storage device housed inor directly attached to a first node and a second physical storagedevice housed in or directly attached to a second node; wherein thedatastore includes stored data that is backed by both the first physicalstorage device and the second physical storage device of the pluralityof physical storage devices; and wherein the stored data is stored bymapping identifiers to the stored data; and enabling creation of a firstrepresentation of a first virtual disk organized in accordance with afirst designated file system and a first defined storage policy; andenabling creation of a second representation of a second virtual diskorganized in accordance with a second designated file system and asecond defined storage policy.
 3. The method of claim 2, wherein thefirst representation includes a first namespace organized in accordancewith the first designated file system.
 4. The method of claim 2, whereinthe second representation includes a second namespace organized inaccordance with the second designated file system.
 5. The method ofclaim 2, wherein, using data striping, a first portion of the firstvirtual disk is stored on the first physical storage device and a secondportion of the first virtual disk is stored on the second physicalstorage device.
 6. The method of claim 2, wherein the respective node ofthe plurality of nodes is a host computer.
 7. The method of claim 2,wherein exposing the abstraction of the datastore includes exposing aglobal namespace.
 8. The method of claim 2, wherein the datastoreincludes metadata corresponding to a portion of the stored data, whereinthe metadata includes information about the location of the portion ofstored data on the plurality of physical storage devices.
 9. The methodof claim 8, wherein the metadata is stored on the first node and thesecond node.
 10. A system for providing access to a plurality ofphysical storage devices, the system comprising: one or more processors;and memory storing one or more programs configured to be executed by theone or more processors, the one or more programs including instructionsfor: executing a virtual computing instance on a respective node of aplurality of nodes, wherein the respective node includes a hypervisor, aplurality of physical storage devices, random access memory, a computerprocessing unit, and a storage management module; exposing, to thevirtual computing instance, an abstraction of a datastore, wherein thedatastore is backed by the plurality of physical storage devices housedin or directly attached to a plurality of nodes including a firstphysical storage device housed in or directly attached to a first nodeand a second physical storage device housed in or directly attached to asecond node; wherein the datastore includes stored data that is backedby both the first physical storage device and the second physicalstorage device of the plurality of physical storage devices; and whereinthe stored data is stored by mapping identifiers to the stored data; andenabling creation of a first representation of a first virtual diskorganized in accordance with a first designated file system and a firstdefined storage policy; and enabling creation of a second representationof a second virtual disk organized in accordance with a seconddesignated file system and a second defined storage policy.
 11. Thesystem of claim 10, wherein the first representation includes a firstnamespace organized in accordance with the first designated file system.12. The system of claim 10, wherein the second representation includes asecond namespace organized in accordance with the second designated filesystem.
 13. The system of claim 10, wherein, using data striping, afirst portion of the first virtual disk is stored on the first physicalstorage device and a second portion of the first virtual disk is storedon the second physical storage device.
 14. The system of claim 10,wherein the respective node of the plurality of nodes is a hostcomputer.
 15. The system of claim 10, wherein exposing the abstractionof the datastore includes exposing a global namespace.
 16. The system ofclaim 10, wherein the datastore includes metadata corresponding to aportion of the stored data, wherein the metadata includes informationabout the location of the portion of stored data on the plurality ofphysical storage devices.
 17. The system of claim 16, wherein themetadata is stored on the first node and the second node.
 18. Anon-transitory computer-readable storage medium storing one or moreprograms configured to be executed by one or more processors of a systemto provide access to a plurality of physical storage devices, the one ormore programs including instructions for: executing a virtual computinginstance on a respective node of a plurality of nodes, wherein therespective node includes a hypervisor, a plurality of physical storagedevices, random access memory, a computer processing unit, and a storagemanagement module; exposing, to the virtual computing instance, anabstraction of a datastore, wherein the datastore is backed by theplurality of physical storage devices housed in or directly attached toa plurality of nodes including a first physical storage device housed inor directly attached to a first node and a second physical storagedevice housed in or directly attached to a second node; wherein thedatastore includes stored data that is backed by both the first physicalstorage device and the second physical storage device of the pluralityof physical storage devices; and wherein the stored data is stored bymapping identifiers to the stored data; and enabling creation of a firstrepresentation of a first virtual disk organized in accordance with afirst designated file system and a first defined storage policy; andenabling creation of a second representation of a second virtual diskorganized in accordance with a second designated file system and asecond defined storage policy.
 19. The non-transitory computer-readablestorage medium of claim 18, wherein the first representation includes afirst namespace organized in accordance with the first designated filesystem.
 20. The non-transitory computer-readable storage medium of claim18, wherein the second representation includes a second namespaceorganized in accordance with the second designated file system.
 21. Thenon-transitory computer-readable storage medium of claim 18, wherein,using data striping, a first portion of the first virtual disk is storedon the first physical storage device and a second portion of the firstvirtual disk is stored on the second physical storage device.
 22. Thenon-transitory computer-readable storage medium of claim 18, wherein therespective node of the plurality of nodes is a host computer.
 23. Thenon-transitory computer-readable storage medium of claim 18, whereinexposing the abstraction of the datastore includes exposing a globalnamespace.
 24. The non-transitory computer-readable storage medium ofclaim 18, wherein the datastore includes metadata corresponding to aportion of the stored data, wherein the metadata includes informationabout the location of the portion of stored data on the plurality ofphysical storage devices.
 25. The non-transitory computer-readablestorage medium of claim 24, wherein the metadata is stored on the firstnode and the second node.